Field of the Invention
This invention relates to the improvement of heat transfer in a marine keel cooler, and in particular to improving heat transfer of the internal coolant flowing through keel cooler coolant tubes.
Discussion of the Prior Art
Heat-generating sources in marine vessels are often cooled by water, other fluids, or water mixed with other fluids. In marine vessels, cooling fluid or coolant flows through the engine or other heat generating source where the coolant picks up heat and then flows to another part of the plumbing circuit. The heat must be transferred from the coolant to the ambient surroundings, such as the body of water in which the vessel is located. For small vessels having outboard motors, the raw ambient water being pumped through the engine is a sufficient coolant. However, as the vessel power demand gets larger, ambient water pumped through the engine serves as a source of significant contamination damage, particularly if the ambient water is corrosive salt water and/or carries abrasive debris.
There have been developed various apparatuses for cooling engines and other heat sources of marine vessels. One such apparatus that uses coolant in a closed-loop plumbing circuit is a keel cooler. Keel coolers were developed more than 70 years ago for attachment to a marine hull structure, an example of which is described in U.S. Pat. No. 2,382,218 (Fernstrum). A keel cooler is basically composed of a pair of spaced headers secured to the hull and separated by a plurality of heat conduction or coolant tubes. In the plumbing circuit of a vessel, hot coolant flows from the engine and into the keel cooler header located beneath the water level (i.e., below the aerated water level), and then into the coolant tubes. The coolant flows through the coolant tubes to the opposite header, and the cooled coolant returns through the plumbing circuit to the engine. The headers and coolant tubes disposed in the ambient water operate to transfer heat from the coolant, through the walls of the coolant tubes and headers, and into the ambient water. The foregoing type of keel cooler is referred to as a one-piece keel cooler, since it is an integral unit with its major components being welded or brazed in place. However, other types of keel coolers are known, including demountable keel coolers having spiral tube configurations wherein the major components, including coolant tubes, are detachable.
An important aspect of a keel cooler is the ability to efficiently transfer heat from the coolant flowing through the inside of the coolant tubes into the cooler ambient water around the outside. There are several factors that impact keel cooler heat transfer, one of which is the rate at which the heat flows into, or out from, either the interior fluid (i.e., coolant) or exterior fluid (i.e., ambient water). A high resistance to heat flow in either fluid will produce a slow overall rate of heat transfer. For the coolant, the inside heat transfer (Hi) is a function of coolant thermal properties, inside tube geometry, coolant flow rate, coolant flow distribution per tube, coolant flow characteristics (i.e., laminar or turbulent), and inside wall friction coefficients. For the ambient water, the outside heat transfer (Ho) is a function of outside fluid thermal properties, outside tube/keel cooler geometry, flow characteristics and restrictions, tube assembly, location on the hull, and speed and direction of ambient water passing over the keel cooler. Other factors to consider in overall heat transfer include the coolant tube wall thickness and the thermal conductivity of the tube material.
One known way to improve overall heat transfer is to increase the effective area of the keel cooler in order to increase the conductive barrier provided for heat flow. In other words, a larger keel cooler area will result in a greater amount of heat that will flow in a given time with a given temperature differential. Keel coolers are usually disposed in recesses at the bottom of the hull of the vessel, and sometimes are mounted on the side of the vessel, but always below the water line. The area on the vessel hull which is used to accommodate a keel cooler is referred to as the “footprint.” However, an important aspect of keel coolers for marine vessels is the requirement that they have as small a footprint as possible, while fulfilling or exceeding their heat exchange requirement and minimizing pressure drops in coolant flow. As such, keel coolers in the prior art have minimized their footprint by utilizing rectangular tubes and spacing them relatively close to each other to create a large heat flow surface area. Accordingly, keel coolers in the prior art often have a total of eight rectangular coolant tubes extending between the two headers, including six intermediate tubes and two outer-side tubes, which usually have cross-sectional dimensions of either 1.375 in.×0.218 in., 1.562 in.×0.375 in., or 2.375 in.×0.375 in. However, demands for improving engine fuel efficiency and payload capacity of vessels have resulted in higher engine output temperatures and a greater demand on keel cooler heat transfer efficiency, and since the keel cooler must maintain as small a footprint as possible, there exists a need to improve the heat transfer efficiency of the keel cooler in other ways.
Another way to improve keel cooler heat transfer is to enhance the flow rate and flow distribution of the internal coolant. It is well known that the flow rate of the coolant flowing through the coolant tubes has a velocity upon which the heat transfer is partially dependent. Moreover, it is also well known in the keel cooler art that the two outer-side tubes have the greatest area of exposure to the external ambient water, and that increasing flow distribution to these outer tubes would also improve keel cooler efficiency. However, keel coolers with rectangular headers and rectangular heat conduction tubes may provide imbalanced coolant flow among the parallel tubes, which can lead to both excessive pressure drops and inferior heat transfer. In particular, coolant flowing through the heat exchanger may have limited access to the outer-side tubes even in the presence of orifices designed for passing coolant to these outer-side tubes. As such, the vast majority of keel cooler developments in the past 15 years have focused on improving heat transfer efficiency by enhancing as well as equalizing the flow rate through the side tubes and intermediate tubes. For example, U.S. Pat. No. 6,575,227 (having the same assignee as the present application) was directed toward a keel cooler having a beveled bottom wall with outer-side tube orifices being in the natural flow path of coolant flow for improving flow rate and flow distribution to the coolant tubes. U.S. Pat. No. 6,896,037 (also having the same assignee) additionally provided in the header a fluid flow diverter for facilitating coolant flow towards both the inner tubes and the outer-side tubes. U.S. Pat. No. 7,055,576 (Fernstrum) was directed toward an apparatus for enhancing keel cooler efficiency by increasing the flow rate of coolant through side tubes by using apertures in an arrow-shaped design. However, as already mentioned, the demand on keel cooler efficiency continues to increase, and there exists a need for a new development in the art of keel coolers, which is satisfied by the present invention.
An approach for improving keel cooler heat transfer that has received no attention in the prior art is through the enhancement of turbulent flow of the internal coolant flowing through coolant tubes. In most modern keel cooler designs, the rectangular coolant tubes have a relatively smooth inner surface that promotes laminar flow of the cooling fluid at or near the coolant tube interior walls. Laminar flow is defined as a flow condition where a viscous fluid flows in contact with a tube surface at a low velocity so as not to produce any intermixing of the fluid. In a laminar flow regime, the fluid in contact with the tube wall will have its velocity reduced by viscous drag or friction, which produces a “boundary layer” that acts as a region of high viscous shear stress. This viscous shear layer, or boundary layer, acts to retard the passage of fluid along the pipe through the no-slip condition at the wall. Within the boundary layer, these viscous, frictional stresses cause energy dissipation into the bulk fluid, which appears as heat. In other words, the boundary layer not only inhibits mixing in the bulk fluid, but also acts as an insulative heat generating layer at the coolant tube interior wall (i.e., the heat transfer surface), therefore reducing the overall heat transfer of the keel cooler.
On the other hand, enhancing turbulence within the coolant can help to minimize the thermally resistant boundary layer. Turbulence is generally defined as the flow regime in which the fluid exhibits chaotic property changes, such as rapid fluctuations in velocity and pressure of the fluid about some mean value. Whether fluid flow will result in laminar or turbulent flow is primarily determined by the Reynolds number, which may be defined as the ratio between the inertial force and viscous force of the fluid. As such, the Reynolds number is a function of the fluid velocity, and as fluid velocity increases, a transition region can be reached in which the inertial forces dominate over the viscous forces. This may allow for the development of turbulent eddies in the fluid which can impact and destroy the boundary layer, resulting in a decrease in boundary layer thickness. As turbulence is further increased, eddying motion can become increasingly unsteady, causing the eddies to burst from the wall and mix with the bulk fluid (i.e., the region of fluid outside of the boundary layer that is further from the tube wall). The turbulent eddies that are formed can transport large quantities of thermal energy. Therefore, heat transfer can be increased where the eddies bursting from and/or impacting the tube wall act to disrupt or destroy the boundary layer insulation and take large amounts of cooler fluid from the wall and distribute it into the hotter bulk fluid regions.
While the science behind turbulence is not considered a well-understood art, it is generally believed that increasing turbulent flow inside of a keel cooler tube will result in an increase in the pressure drop of the coolant. This is believed to be caused by the turbulent eddies of various sizes interacting with each other as they move around, exchanging momentum and energy, and consuming the fluid's mechanical energy as the bulk fluid is forced to drive these unsteady eddy motions. In other words, in the keel cooler art, it is believed that enhancing turbulence will result in increased drag and pressure drop due to the increased transverse motion of fluid particles that oppose the direction of bulk fluid flow. In the keel cooler art, increasing system pressure drop is considered devastating to keel cooler performance and detracts from the overall usefulness of the keel cooler. This is because keel coolers on marine vessels are generally limited by the pumping capacity of the marine motor and do not usually have external pumps that can compensate for increased pressure drop. In other words, unlike land-based heat exchanger systems that can accommodate larger footprints with external pumps, keel coolers have strict size and payload constraints that practically preclude the use of an external pump. It is for this reason that developments in the keel cooler art have traditionally avoided enhancing coolant turbulence, for concerns over increasing pressure drop.
The only known keel cooler on the market that allegedly attempts to disrupt the coolant flow pattern inside of a rectangular keel cooler tube is an apparatus having a plurality of roughness elements on the interior surface of the coolant tube. The roughness elements of this known apparatus are small protrusions in the form of bumps arranged on the coolant tube interior wall. The bumps of this apparatus are about 0.015 inches in height, with a diameter of 0.022 inches, and spaced evenly by 0.060 inches in a staggered configuration. It is believed that the purpose of these roughness elements is to disrupt the boundary layer insulation at the coolant tube interior wall. However, it is well known in the keel cooler industry that this apparatus significantly increases pressure drop with de minimus improvement in heat transfer. Therefore, it is believed that this device does not enhance turbulent coolant flow and/or generate unsteady eddying motions as to effectively mix the bulk coolant to improve heat transfer. Instead, this apparatus acts to increase surface roughness of the coolant tube wall, which increases the friction factor according to the well-known Moody diagram, and therefore results in the observed increase in pressure drop. The introduction of this apparatus into the keel cooler market has only further detracted those skilled in the art from pursuing coolant flow characteristics as an avenue for successfully increasing heat transfer.
As it generally pertains to keel cooler heat transfer, there are known keel coolers of only general interest that use external fins to improve the outside heat transfer (Ho) with the ambient water. For example, U.S. Pat. No. 3,841,396 (Knaebel) provides for a marine vessel heat exchanger having a series of radially extending external fins connected to a longitudinal member. The Knaebel invention provides these external fins to increase the surface area of the heat exchanger and does not teach turbulent flow to improve internal heat transfer (Hi). In U.S. Pat. No. 3,240,179 (Van Ranst), a marine heat exchanger is disclosed providing a bottom sheet portion in a transverse sinuous configuration. The Van Ranst invention is intended to provide a relatively large effective heat exchange area in proportion to the complete unit. The Van Ranst invention further provides for a smooth flow path of the inner coolant fluid, which is described as “optimal” and is believed to teach away from promoting turbulent fluid flow. In U.S. Pat. No. 3,650,310 (Childress), a combination boat trim tab and heat exchanger is provided having elongated fins secured to the bottom of the outside of the body to increase heat exchange area. Childress further provides an internal serpentine passageway and internal cooling fins to further increase the heat exchange area between the cooling liquid and the body. The invention in Childress does not disclose the use of turbulent coolant flow to increase heat transfer. U.S. Pat. No. 3,177,936 (Walter) provides a marine heat exchanger that includes a fluted heat exchange tube with an internal helical baffle. The fluted tube of the Walter invention is intended to increase heat exchange surface area, as well as improve the flow of external seawater over the tubes. The helical baffle in the Walter invention is intended to mechanically agitate the coolant and to partition the tubes into at least two stream passages of a serpentine form. The Walter invention does not disclose promoting turbulent flow of the coolant, as this term was well known in the art at the time of that invention. More particularly, Walter does not teach enhancing turbulence through naturally occurring eddying motions to improve bulk fluid mixing, and instead merely mechanically agitates the coolant to some unknown degree. Moreover, such partitioning inside of the coolant tube is believed to restrict coolant flow, which would result in a substantial increase in pressure drop compared to a similarly situated tube without the flutes and baffle. Therefore, as can be seen by these shortcomings in the keel cooler prior art, there exists a need to further improve heat transfer without increasing pressure drop, which can be achieved by the present invention through the provision of turbulence enhancers for use in the internal coolant.
Turbulators, which are known as inserts, tube inserts, impediments, or static mixers, are known to be arranged inside of a tube in order to promote and/or enhance turbulent fluid flow. Although turbulators are known to enhance turbulence and promote bulk fluid mixing to improve heat transfer, they are also known to detrimentally increase pressure drop. Because those skilled in the keel cooler art have been taught to avoid increased pressure drop due to the pumping constraints of marine motors, the use and teachings of turbulators have generally been confined to land-based heat exchanger systems where pressure loss can be compensated by external pumping means. Moreover, the relatively slow rate of innovation in the keel cooler art, combined with the lack of understanding of turbulence, has only further detracted those persons with ordinary skill in the keel cooler art from logically commending their attention to other heat exchanger systems.
Accordingly, there have been various patents of only general interest pertaining to turbulators which have issued over the years. U.S. Pat. No. 3,981,356 (Granetzke) describes a heat-exchange tube with a strip of expanded metal arranged in a helix to form a turbulator. This arrangement is alleged to direct a portion of the liquid toward the inner wall surface to control heat flow, however, it also results in increased pressure drop. The Granetzke invention alleges to regulate this increase in pressure drop by modifying the expanded metal configuration. Referring next to U.S. Pat. No. 6,578,627 (Liu et al.), this patent discloses a fin-pattern of ribbed vortex generators for an air conditioner system having a plurality of prism-like structures on the fin. The structures have different heights for improving heat transfer while allegedly causing little pressure drop-off. Similarly, U.S. Pat. No. 7,637,720 (Liang) provides a turbulator for use with a turbine blade of a gas turbine engine having an inverted V-shape with a diffusion slot between adjacent turbulators. In U.S. Pat. No. 4,865,460 (Friedrich), a static mixing device is disclosed having a plurality of rows of spaced parallel tubes extending across the conduit. The tubes are arranged so that adjacent tubes are located at right angles to each other, which provides a tortuous path for the viscous resin medium to be mixed. The Friedrich invention requires the product to be fed through the tortuous path of the static mixer at “high pressure,” and does not disclose the effect of pressure loss.
In light of the foregoing, it should be understood that keel coolers with the smallest footprint, greatest overall heat transfer, and least internal pressure drop are considered the most desirable. However, despite the various efforts to enhance turbulence and increase heat transfer using turbulators in general heat exchangers, there has been no known development in this area with respect to marine keel coolers. The demand on keel cooler efficiency is increasing as marine motors must become more efficient and carry heavier payloads. If turbulence enhancers can be selected to increase heat transfer while not substantially increasing pressure drop to an unacceptable level, there could be significant economic savings in the keel cooler industry. Therefore, there exists a long-felt, yet unsatisfied need for a keel cooler that improves heat transfer by enhancing turbulent coolant flow inside of the coolant tubes without a substantial increase in pressure drop. Such a keel cooler with improved heat transfer could further reduce the size required of the keel cooler, the cost of acquiring keel coolers, and the manufacturing costs associated with keel coolers.